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jdb(1)                                                                  jdb(1)




NAME

       jdb - Java debugger


SYNOPSIS

       jdb [ options ] [ class ] [ arguments ]


PARAMETERS

       options        Command-line options.

       class          Name of the class to begin debugging.

       arguments      Arguments passed to the main() method of class.


DESCRIPTION

       The  Java  debugger,  jdb,  is  a simple command-line debugger for Java
       classes.  It is a demonstration of the Java Platform Debugger Architec-
       ture  that  provides inspection and debugging of a local or remote Java
       Virtual Machine.

   Starting a jdb Session
       There are many ways to start a jdb session.  The most  frequently  used
       way is to have jdb launch a new Java Virtual Machine (VM) with the main
       class of the application to be debugged.  This is done by  substituting
       the  command jdb for java(1) in the command line.  For example, if your
       application's main class is MyClass, you use the following  command  to
       debug it under jdb:

              example% jdb MyClass

       When  started this way, jdb invokes a second Java VM with any specified
       parameters, loads the specified class, and stops the VM before  execut-
       ing that class's first instruction.

       Another  way to use jdb is by attaching it to a Java VM that is already
       running.  A VM that is to be debugged with jdb must be started with the
       following options:


       +-------------------+--------------------------------------+
       |      option       |               purpose                |
       +-------------------+--------------------------------------+
       |-Xdebug            | Enables debugging support in the VM. |
       +-------------------+--------------------------------------+
       |-Xrunjdwp:trans-   | Loads in-process debugging libraries |
       |port=dt_socket,    | and specifies the kind of connection |
       |server=y,suspend=n | to be made.                          |
       +-------------------+--------------------------------------+
       For example, the following command will run the MyClass application and
       allow jdb to connect to it at a later time:

              example% java -Xdebug \
              -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n \
              Class

       You can then attach jdb to the VM with the following command:

              example% jdb -attach 8000

       Note that MyClass is not specified in the jdb command line in this case
       because  jdb is connecting to an existing VM instead of launching a new
       one.

       There are many other ways to connect the debugger to a VM, and  all  of
       them are supported by jdb.  The Java Platform Debugger Architecture has
       additional documentation on these connection options.

   Basic jdb Commands
       The following is a list of the basic jdb commands.  The  Java  debugger
       supports other commands listed with the help command.

       Notice  that  to  display  local (stack) variables, the class must have
       been compiled with the javac -g option.

       cont           Continues execution of the debugged application after  a
                      breakpoint, exception, or step.

       dump           For  primitive  values,  this  command  is  identical to
                      print.  For objects, it prints the current value of each
                      field defined in the object.  Static and instance fields
                      are included.

                      The dump command supports the same set of expressions as
                      the print command.

       help, or ?     As  the  most  important  jdb command, help displays the
                      list of recognized commands with a brief description.

       print          Displays Java objects and primitive values.   For  vari-
                      ables  or fields of primitive types, the actual value is
                      printed.  For objects, a short description  is  printed.
                      See  the dump command for getting more information about
                      an object.

                      print supports many simple  Java  expressions  including
                      those with method invocations.  For example:

                      o  print MyClass.myStaticField
                      o  print myObj.myInstanceField
                      o  print i + j + k  ... where i, j, and k are primitives
                         and either fields or local variables.
                      o  print myObj.myMethod()  ...  if  myMethod  returns  a
                         non-null.
                      o  print new java.lang.String("Hello").length()

       thread         Selects  a  thread  to  be the current thread.  Many jdb
                      commands are based on the setting of the current thread.
                      The  thread is specified with the thread index described
                      in the threads command.

       threads        Lists the threads that are currently running.  For  each
                      thread, its name and current status are printed, as well
                      as an index that can be used for  other  commands.   For
                      example:

                           4. (java.lang.Thread)0x1 main running

                      In this example, the thread index is 4, the thread is an
                      instance of java.lang.Thread, the thread name  is  main,
                      and it is currently running

       run            After  starting  jdb,  and  setting any necessary break-
                      points, use this command to start the execution  of  the
                      debugged  application.   This  command is available only
                      when jdb launches the debugged application  (as  opposed
                      to attaching to an existing VM).

       where          The  where  subcommand with no arguments dumps the stack
                      of the current thread (which is set with the thread com-
                      mand).   Using  where all dumps the stack of all threads
                      in the current thread group.   Using  where  threadindex
                      dumps the stack of the specified thread.  If the current
                      thread is suspended (either through an event such  as  a
                      breakpoint  or through the suspend command), local vari-
                      ables and fields can be displayed  with  the  print  and
                      dump  commands.   The  up and down commands select which
                      stack frame is current.

   Breakpoint Commands
       Breakpoints are set in jdb at line numbers or at the first  instruction
       of a method.  For example:

       stop at MyClass:22       Sets a breakpoint at the first instruction for
                                line 22 of the source file containing MyClass.

       stop in java.lang.String.length
                                Sets  a  breakpoint  at  the  beginning of the
                                method java.lang.String.length.

       stop in MyClass.init     init identifies the MyClass constructor.

       stop in MyClass.clinit   clinit identifies  the  static  initialization
                                code for MyClass.

       If  a method is overloaded, you must also specify its argument types so
       that the proper method can be selected for a breakpoint.  For example,

                      MyClass.myMethod(int,java.lang.String)

       or

                      MyClass.myMethod()

       The clear command removes breakpoints using a  syntax  as  in  clearMy-
       Class:45.   Using the clear command with no argument displays a list of
       all breakpoints currently set.  The cont command continues execution.

   Stepping Commands
       The step command advances execution to the next line, whether it is  in
       the  current stack frame or a called method.  The next command advances
       execution to the next line in the current stack frame.

   Exception Commands
       When an exception occurs for which there is no catch statement anywhere
       in  the  throwing thread's call stack, the VM normally prints an excep-
       tion trace and exits.  When running under jdb, however, control returns
       to  jdb  at the offending throw.  Use jdb to determine the cause of the
       exception.

       catch          Causes the debugged application to stop at other  thrown
                      exceptions.  For example:

                           catch java.io.FileNotFoundException

                      or

                           catch mypackage.BigTroubleException

                      Any  exception  which  is  an  instance of the specified
                      class (or of a subclass) will stop  the  application  at
                      the point where it is thrown.

       ignore         Negates  the effect of a previous catch command.  Notice
                      that the ignore command does not cause the  debugged  VM
                      to ignore specific exceptions, only the debugger.


OPTIONS

       When using jdb in place of the Java application launcher on the command
       line, jdb accepts many of the same  options  as  the  java(1)  command,
       including -D, -classpath, and -Xoption.

       The following additional options are accepted by jdb:

       -sourcepath dir1:dir2:...
                      Uses the given path in searching for source files in the
                      specified path.  If this option is  not  specified,  the
                      default path of "." is used.

       -attach address
                      Attaches the debugger to previously running VM using the
                      default connection mechanism.

       -launch        Launches  the  debugged  application  immediately   upon
                      startup  of jdb.  This option removes the need for using
                      the run command.  The debuged  application  is  launched
                      and  then  stopped  just  before the initial application
                      class is loaded.  At that point, you can set any  neces-
                      sary  breakpoints  and  use the cont command to continue
                      execution.

       -J  option     Pass option to the Java virtual machine, where option is
                      one  of  the  options  described on the man page for the
                      java application launcher,  java(1).  For  example,  -J-
                      Xms48m  sets the startup memory to 48 megabytes. It is a
                      common convention for -J to pass options to the underly-
                      ing virtual machine.


       Other options are supported for alternate mechanisms for connecting the
       debugger and the VM it is to debug.  The Java Platform Debugger  Archi-
       tecture  has additional documentation on these connection alternatives.


SEE ALSO

       java(1), javac(1), javadoc(1), javah(1), javap(1)




                                  12 Nov 2001                           jdb(1)

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